1. Field of the Invention
The present invention relates to apparatus for protecting aircraft radomes, antennas, and sensitive electronic equipment in aircraft from damage due to lightning strikes. More particularly, the present invention relates to lightning diversion strips that are placed on radomes or other dielectric surfaces, and, when exposed to the strong electrical fields associated with lightning, form ionized channels that conduct the current of the lightning strike directly to the metal structure of the aircraft and away from the structure and the sensitive electronic equipment housed within the radome or other dielectric structure.
2. Description of the Related Art
The operation of an exemplary lightning protection system for the radome of an aircraft, or the like, is described in U.S. Pat. No. 3,416,027 (hereinafter "the '027 patent"), the disclosure of which is incorporated herein by reference. As set forth in the '027 patent, a lightning diversion strip consists of a series of metallic or conductive segments that are positioned on a strip assembly. The complete assembly is then applied to the radome or another dielectric structure of an aircraft. The strip assemblies are electrically connected to the frame or to the metallic outer skin of the aircraft. The conductive segments are spaced apart on the strip so that the segments are separated by gaps having air or another dielectric therein.
In the presence of a high voltage field, such as that associated with a lightning strike, the air above and between the conductive segments ionizes to provide a conductive path for the electromagnetic energy of the lightning to the frame or skin of the aircraft. The lightning strike is thereby diverted across the external surface of the structure and prevented from puncturing through the wall and damaging the structure and the sensitive electronic equipment that is beneath the surface of the radome or other portion of the aircraft. Upon completion of the lightning strike process, the air in the vicinity of the conductive segments returns to the normal, non-ionized state, and will remain in that condition until the next lightning strike process is initiated. Since the high currents caused by a lightning strike are conducted to the frame or skin of the aircraft through an ionized air channel located above the metallic segments rather than through the individual metallic segments, the metallic segments remain substantially unaffected by the lightning strike and their replacement is normally not necessary, even after multiple strikes.
In typical embodiments of devices built in accordance with the '027 patent, the metallic or conductive segments of the strip assemblies are connected by an appropriate resistance material. For example, a typical resistance material has a resistance of 80,000 ohms per foot, or greater, depending upon the application. The resistance material assists in the initiation and establishment of an ionization channel or path in the air above and between the metallic segments during a lightning strike. The resistance material also helps prevent corona discharge and sparking between the metallic segments during atmospheric electrostatic and triboelectric charging (transfer of charge by particle impingement) of the aircraft during inclement weather operation, thereby preventing p-static noise interference in communication and navigation systems aboard the aircraft. Such phenomenon, if permitted to occur, would generate radio frequency noise that interferes with communication and navigation systems located aboard the host aircraft.
In a typical prior art device, the size, shape, and location of the conductive metallic segments in a strip assembly are selected in accordance with the electrical characteristics of the underlying equipment. For example, for radomes that enclose weather radar, the lightning protection strip assemblies must be transparent to the radar transmission and reception. In such cases, the maximum dimension of the metallic segments preferably should be less than approximately one-eighth the wavelength of the highest operating frequency of the radar antenna, (i.e., one-eighth of the shortest wavelength), to prevent the segments from re-radiating and causing electromagnetic interference. The spacing between the metallic segments is selected to be great enough to prevent sparking between the segments during p-static conditions, yet close enough together to establish an ionization channel above and between segments during lightning strikes. The geometric shape of the individual metallic segments may also vary, including, but not limited to, circular, square, oval, diamond, and triangular.
The '027 patent discloses a number of techniques for constructing such lightning protection strip assemblies. For example, FIG. 2 of the '027 patent discloses a strip assembly wherein individual rivets, having appropriate dimensions and spacing, are mounted to the strip assembly. FIG. 3 of the '027 patent illustrates an embodiment wherein copper wire segments, having a selected length and spacing, are applied to a neoprene layer, while the neoprene is still tacky. FIG. 4 of the '027 patent illustrates another embodiment in which metallic plate segments are mounted in a beveled channel. FIG. 5 of the '027 patent illustrates an embodiment wherein metallic segments are formed by spraying a layer of metal on a base strip, over longitudinally spaced rectangular areas.
Formation of the ionized channel is critical to the effectiveness of the strip since lightning diversion only occurs subsequent to channel formation. Lightning diversion strips built in accordance with the '027 patent have proven to be effective in creating such channels, and thereby substantially reducing the damage to dielectric structures and electronic equipment protected by such strips. Strip construction has gradually evolved so that the strips are presently being manufactured using etching techniques similar to those used in the manufacture of printed circuit boards. Such techniques enable a closer spacing between the metalized segments, which in turn causes the air above and between the metalized segments to ionize at a more favorable, i.e., lower voltage, and thus earlier in the lightning event.
When the lightning stepped leader approaches the aircraft during the preliminary phase of a lightning strike, the rapidly increasing high voltage field causes the formation of a number of different streamer channels or potential lightning pathways. The path of the lightning strike is determined by the first pathway that connects with the approaching step leader, and thus completes the "circuit." When ionization between the segments occurs at a lower voltage, the ionization path itself is formed or created in a much shorter time, thereby increasing the likelihood that the lightning strike will pass along the diverter channel rather than connecting with a streamer created within the structure, which would then cause the lightning current to pass through the radome to the sensitive electronic equipment enclosed therein. It is thus this "competition" between various electrical pathways that will determine the efficacy of a diversion system.
Etching of the copper is initiated at the upper, exposed surface, and is continued until the lower surface is breached. In order to assure that there is at least a minimum separation between copper segments, the etching of the lower copper surface is critical. However, for purposes of overall strip operation, it is the distance between the top surfaces of adjacent segments that will determine the electrical field required to establish the ionized channel. Known etching techniques for copper-clad strip assemblies are not able to sufficiently control the strip separation dimensions at both top and bottom surfaces. Consequently, the present strip assemblies are manufactured in a manner wherein the lower surface separation is deemed to be the critical factor. In order to assure achievement of a minimum gap size between individual lower conductive segments, the maximum gap size at the upper surface will vary considerably over the length of a diversion strip. The larger gap sizes in the top surface result in a higher breakdown voltage between affected segments, with all of such segments combining to create a higher breakdown voltage over the length of the diversion strip. Exemplary lightning diversion strips built in accordance with conventional techniques break down at approximately 3,000 volts per inch, whereas it is preferable that the breakdown occur in the presence of a much lower voltage field (e.g., approximately 1,000 volts per inch). Thus, a need exists for an improved lightning diversion strip having a lower breakdown voltage, and which can be constructed using techniques that lend themselves to mass production of the strips.